Spectroscopy is the process of separating energies from samples that characterize said samples. In most cases, other than samples that have naturally emitting energies, such as radioactivity and chemi-illuminsance, it is necessary to irradiate the sample with some form of energy that interacts with the sample to cause it to absorb or emit energies that can be detected in the form of a spectrum. Transmission, Reflection and Absorption spectroscopy are the simplest examples of this interaction between the irradiation energy and the resulting energy spectrum from the sample. In the case of Transmission spectroscopy, only those energies that pass through the sample are detectable in a spectrum. With Absorption spectroscopy, it is the absorbed energies that are detectable in the spectrum. In Reflectance spectroscopy, the detectable energies are those that are reflected from a surface while all others are absorbed.
Fluorescence and Raman spectroscopy are more complex techniques since the irradiating energy interacts with the molecular structure exciting the electrons in such a way that the sample emits energies different and non-complimentary from the irradiating source. This kind of description can even be applied to Nuclear Magnetic Resonance spectroscopy where the radiating energy is a spectrum of different magnetic field strengths and the sample spectrum is the result of the spectrum of the resonating atoms of the sample.
In all spectroscopy in which the sample is not naturally emitting energy, the illuminating energy is to some degree, carried along into the resulting spectrum of the sample. For example, in Absorption spectroscopy, especially in the case of infrared spectroscopy, the baseline of the resulting spectra is the illuminating infrared energy where the molecular structure of the sample absorbs the illuminating at specific wavelengths to provide the characteristic absorption spectrum of the sample. In the case of Reflectance spectroscopy, the reflected spectrum of energies from the surface of the sample is superimposed onto the illumination energies.
Where there is a shift in emission energies from the illumination energies, as with Fluorescence and Raman spectroscopy, narrow wavelength energies, provided by lasers, band pass filters and/or notch filters, are used in an attempt to eliminate the illumination energies. This is done, since in many cases, the energy shift is small and the emission energy may be completely masked by the overlap and strength of the illumination energy. In these cases, knowledge of the spectral characteristics of the sample is required, as well as a very specific instrument setup in order to isolate and optimize the emission energies of the sample from the illumination energies.
As can be appreciated, illuminating energy is the key component in obtaining most absorption and emission spectra since either the illumination is absorbed, or it excites the electrons in the sample to a different energy state and when they fall back to their normal state, energy is emitted. However, both absorption and emission of energy characteristics of the illumination may be present in the resulting sample spectra. For example there are strong peaks in illumination spectra when produced from, for example, mercury arc and xenon arc sources. In most synthetic, as well as natural illumination sources, there are absorption bands that are characteristic of the elements producing the illumination, such as from halogen lamps and sunlight or starlight. These extra spectral contributions from the illumination can interfere with the identification of the spectral components from the sample. Usually, filters are used to remove these artifacts. However, the use of filters may also limit the response of the sample and its spectral components. Pure sample spectra would result if the illumination component could be eliminated without the use of filters.
Most samples have a multitude of spectral parameters that characterize and identify them and their composition. However, each form of spectroscopy requires a different instrument that processes the illumination and emission energies in different specific ways to optimize the signal to noise ratio of the sample for that specific form of spectroscopy. It would be convenient, efficient, and more informative, if there were a way to obtain spectral data from a single instrument that presents a single spectrum that represented the combine spectroscopes.